Inductor at the start of a circuit.... Why/what purpose?

[si2030]

Hi there,

I am looking at a switchmode power design and noticed this double inductor which appears to be between the in plug and the rest of the circuit... I have taken a picture of it in the circuit and also another very similar but bigger one on the bench. These are ringed in black and the board input is in red. If you look closely at the circuit board you can see the (+) and (-) tracks under the board and the inductors are part of these circuit tracks.. (the IC is a TL494).

View attachment 61676

The bigger one on the bench has an inductance of 22mH.

I was hoping someone might shed some light on why this is in the circuit and also what its purpose is??

Would it be in the AC or DC part of the circuit?

I have a number of these wrecked from circuits I had from a recycle yard and wondered if its a good idea to include one of these in a switchmode bench power supply I am building?


Hope someone might answer these questions.

Kind Regards

Simon

 

[JimB]

In that application they are sometimes referred to as a "common mode choke".

A switchmode powersupply is inherently noisy in a electrical sense. Due to the high frequency oscillator used to do the switching, high frequency noise can be conducted out of the PSU and cause wideband interference to radios around the power supply.

Those chokes, which can be on the input and the output of the PSU are intened to keep the electrical noise inside the power supply where it will not cause any problems.

JimB